Flame retardation of thermoplastic resins such as styrene resins and polyester resins have conventionally been carried out by compounding of flame-retardants, e.g., organic halogen compounds, and, if desired, flame-retardant assistants, e.g., antimony trioxide. Typical organic halogen compounds as flame-retardants include tetrabromobisphenol A (TBA) and decabromodiphenyl ether (DBDPE).
TBA, when compounded into thermoplastic resins, brings about great reductions in heat resistance and thermal stability and also a reduction in weatherability. DBDPE, when compounded into thermoplastic resins, causes a considerable reduction in weatherability. Besides, DBDPE has an extremely high melting point of about 307.degree. C. and is therefore poor in dispersibility or compatibility in or with the resins, resulting in reductions in mechanical strength and appearance of molded articles. Further, these conventional flame-retardants are attended by corrosion of molds due to evolution of a halogen gas.
In order to settle down these problems, it has been proposed to compound a flame-retardant comprising a halogenated bisphenol A type epoxy resin into a thermoplastic resin as disclosed in JP-A-61-241343 and JP-A-62-4737 (the term "JP-A" as used herein means an "unexamined published Japanese patent application").
However, these flame-retardants turned to impair thermal stability of thermoplastic resins during molding. That is, the thermal history of the thermoplastic resin in the molten state during kneading in an extruder or molding in an injection molding machine induces decomposition chiefly of the flame-retardant itself, which tends to lead to not only deteriorated appearances of molded articles due to discoloration or generation of a burned mark, but also reductions in mechanical strength and thermal resistance of molded articles. Particularly serious reduction in thermal stability is observed with styrene resins and polyester resins which are molded at relatively high temperatures.
In an attempt to obtain a thermally stable flame-retardant thermoplastic resin composition, the inventors compounded the above-described flame-retardant resin composition with a known phenolic antioxidant as a heat stabilizer but only to provide a resin composition suffering from discoloration or burning, failing to accomplish the purpose.
A flame-retardant resin composition containing a brominated aliphatic hydrocarbon flame-retardant in combination with a specific organic phosphite compound is known as a flame-retardant composition having improved thermal stability as disclosed in JP-A-61-106641. However, because the flame-retardant used here has a low decomposition temperature, the effect of improving thermal stability is virtually lost when the resin composition is molded generally at temperatures exceeding 230.degree. C.